Long Term Evolution (LTE) is currently under discussion as a next generation mobile communication system of the UMTS system. LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink. The 3GPP work on LTE is also referred to as E-UTRAN (Evolved Universal Terrestrial Access Network). In order to support high data rates, LTE allows for a system bandwidth of up to 20 MHz. LTE is also able to operate in different frequency bands and can operate in at least Frequency Division Duplex (FDD) and Time Division Duplex (TDD). The modulation technique or the transmission scheme used in LTE is known as OFDM (Orthogonal Frequency Division Multiplexing). LTE-advanced can be viewed as a future release of the LTE standard and since it is an evolution of LTE, backward compatibility is important so that LTE-advanced can be deployed in spectrum already occupied by LTE. In both LTE and LTE-advanced radio base stations known as eNBs or eNodeBs—where e stands for evolved—, multiple antennas with beamforming technology can be adopted in order to provide high data rates to user equipments. Thus, LTE and LTE-advanced are an example of Multiple-Input-Multiple-Output (MIMO) radio systems. Another example of a MIMO based system is Worldwide Interoperability for Microwave Access (WiMAX) system.
As defined in 3GPP LTE, RS is multiplexing with data into OFDM time-frequency grid. In Rel-8, three kinds of downlink reference signals are defined in terms of different functionality, i.e. cell-specific reference signal, UE-specific reference signal and MBSFN reference signal. Here, the first two kinds of reference signals are focused.
Cell-specific reference signals (also called CRS) are responsible for both channel measurement and data demodulation in Rel-8 for the downlink transmission modes except mode 7 (i.e. single layer beamforming). UE terminals will use CRS to calculate Channel Quality Indication/Precoding Matrix Index/Rank Indicator (CQI/PMI/RI) and also do data demodulation by performing channel estimation. Total 4 CRS are defined to support up to 4 layers transmission, i.e. antenna ports 0-3, as shown in FIGS. 1A, 1B, 1C (normal CP only), the detailed description can be found in the standard document, “3GPP TS 36.211 V8.6.0 (2009-03)”.
UE-specific reference signals (also called dedicated reference signal, or DRS) are introduced only for the transmission mode 7 in Rel-8, i.e. single layer beamforming. DRS are precoded and share the same precoder as data. In addition, DRS is responsible for channel demodulation only, which means channel estimation for data demodulation will be performed based on DRS. Only single DRS port is defined in Rel-8 to support single layer transmission, i.e. antenna port 5. The DRS pattern for normal CP and extended CP is defined respectively, as shown in FIG. 2 (normal CP only). The detailed description can be found in the standard document, “3GPP TS 36.211 V8.6.0 (2009-03)”.
In “3GPP, RAN1—58bis Chairman notes, RAN1—58bis, Oct. 12-16, 2009, Miyazaki, Japan”, DM-RS design for normal CP, as shown in FIG. 3, acts as up to four layers DM-RS pattern. Some characteristics are summarized here: total 12 resource elements per layer are agreed as DM-RS overhead (either marked as 1 or 2 in FIG. 3). Two CDM groups (marked as 1 and 2) occupy different subcarriers in frequency domain. Each CDM group uses 6 orthogonal code covers (OCC) with each length of 2 to multiplex up to two layers. This is a non-staggered version, i.e. 1st DM-RS cluster and 2nd DM-RS cluster take the same subcarrier.
In order to fulfill LTE-advanced downlink spectral efficiency, 30 bps/Hz, up to 8 layer transmission should be supported using some kind of advanced antenna configuration, e.g. 8×8 high-order MIMO. If similar design of up to 8 cell-specific reference signals is introduced, system overhead will be inevitably increased and accordingly throughput performance will be decreased seriously. Accordingly, there is a need for generating a DM-RS pattern for extended CP supporting from rank 1 to rank 8 for both normal subframe and Downlink Pilot Time Slot (DwPTS). Based on this consideration, it is proposed CSI-RS (namely cell-specific reference signal) is targeted for channel measurement only while DM-RS (namely UE-specific reference signal) is targeted for channel demodulation only. CSI-RS can be accessed by all UE terminals in a cell and DM-RS can be accessed by the UE only on the allocated resource blocks (RB).
On DM-RS pattern for extended CP, it is known that extended CP is not supported in conjunction with transmission mode 8 (i.e., dual layer beamforming mode). The use of extended CP is expected for channels with larger time dispersion, or equivalently for channels experiencing much more frequency selectivity as compared to channels where normal CP is used, e.g. Vehicular B channel (VehB). Therefore, there is a need for generating a DM-RS pattern for extended CP to guarantee and improving detection performance in such a channel.
In TDD, DwPTS has different length according to different special subframe configurations, which results in quite a lot of designs at UE side. Therefore, there is a need for generating a DM-RS pattern for extended CP to reduce UE implementation complexity.
The pattern designed for normal CP has been accepted well. Some design principles have been settled down. Therefore, there is a need for generating a DM-RS for extended CP which has smooth and close design for extended CP to further save UE implementation and standard effort.